FIG. 1 is a conceptual diagram illustrating a system for providing a multicast broadcast service in a wireless access system.
Referring to FIG. 1, the multicast broadcast system includes a Multicast and Broadcast Service (MBS) media server 11, an MBS distribution server 12, base stations (BSs) 13a, 13b, and 13c, and mobile stations (MSs) 14a and 14b. 
In this case, the mobile station (MS) is a generic term of electronic appliances capable of wirelessly communicating the base station (BS). The mobile station (MS) may include all kinds of devices having mobility, for example, a mobile terminal or a user equipment (UE). The MBS media server provides base stations (BSs) with MBS data, and performs MS authentication (or UE authentication) for MBS media or encryption key distribution. The MBS distribution server 12 schedules MBS data provided to several base stations (BSs). Optionally, the MBS distribution server may be omitted or the MBS media server may schedule the MBS data. The base station (BS) receives MBS data over a backbone network, and provides the MS or UE with the MBS data via a wireless interface. In this case, the mobile station (MS) may receive the MBS data from the base station (BS).
The MBS for use in the above-mentioned conventional wireless access system can be mainly classified into first and second MBS service types. The first MBS service type is a single-BS access scheme, which is capable of being provided by a single BS within a single cell. The second MBS service type enables several BSs to make a single MBS zone, such that it provides the MBS zone with necessary services using a single MBS zone identifier (ID). For example, the second MBS service type receives the same data burst from the MBS server in several cells belonging to the MBS zone, such that it provides services. Therefore, although the mobile station (MS) moves from one cell to another cell in a single MBS zone, the mobile station (MS) can receive necessary services without a handover. That is, the above-mentioned second MBS service type is a multi-BS access scheme, which can acquire MBS-service continuity and a macro-diversity gain.
The above single-BS access scheme enables a single base station (BS) to provide the MBS service. For example, the single base station (BS) equally distributes a connection ID (CID) allocated to the MBS multicast to all MSs or UEs, such that it provides the MBS service. The multi-BS access scheme enables several base stations (BSs) belonging to the single MBS zone to provide the MSs or UEs with the multicast service using a multicast CID.
The wireless access system implements data communication between the BS and the MS (or UE) according to a service flow. This service flow includes a service flow identifier (SFID), a connection ID (CID), and a quality of service (QoS) parameter. In this case, the SFID identifies a corresponding service flow between the BS and the MS. The CID identifies a connection capable of delivering service flow traffic. The QoS parameter indicates a quality of the service flow.
A general service flow connection is located between the BS and the MS on a one-to-one basis. However, the MBS provides a point-to-multipoint service, which transmits data from a single source to several receivers. Therefore, the MBS service provides several receivers with the same data over a common radio frequency (RF) channel so as to effectively use radio resources.
The base station (BS) transmits the same MBS data to several mobile stations (MSs) via a single service flow. That is, when the MBS service flow is generated between the BS and the MS, the BS assigns the same multicast CID to several mobile stations (MSs), each of which requests reception of MBS data. Therefore, one or more MSs can simultaneously receive the MBS data using the same multicast CID.
Presently, the MBS service enables a single BS to equally assign a predetermined CID to all MSs, each of which desires to receive MBS data. Therefore, although the MS moves from one BS to another BS in the MBS zone, it continuously receives the same MBS data without any handover.
However, a method for performing a handover between MBS zones according to the multi-BS access scheme has not been well known in the art. For example, if the MS moves from a first MBS zone to a second MBS zone, the first and second MBS zones may have different parameters assigned to the MBS service. For example, the first and second MBS zones may have different multicast CIDs. Therefore, the MS must be re-established in the MBS server of a new MBS zone, such that it can receive a desired service within a target MBS zone. Also, the loss of conventional MBS data may unavoidably occur. That is, the MS is unable continuously receive the conventional MBS data.
When a mobile station (MS), which is receiving real-time MBS contents (or real-time MBS content data), enters a new MBS zone during a handover process from an old MBS zone to the new MBS zone, the MS is unable to pre-recognize services provided from the new MBS zone. Therefore, the loss of received MBS packet data may unavoidably occur.